while (my $row = $dbh -> fetchrow_arrayref) {
    if ($row -> [$last_name_col] =~ /^[L-Z]/) {
        push @L_Z => $row;
        while (my $row = $dbh -> fetch_arrayref) {
            push @L_Z => $row;
        }
        last;
    }
    push @A_K => $row;
}